Equipment for handling or processing X-ray film over the years has been improved to transport and process film sheets at ever faster rates. Due to this requirement for high speed operation, the flatness of the individual sheets has become more important during transporting and processing. This is because the sheets encounter various equipment features such as fixed height edge guides, nip conveyance rollers, suction cup handling devices and the like which depend to greater or lesser degrees on the presence of sheets of good flatness. Similar requirements for flatness are found in transporting and handling equipment for other sheet materials, such as photographic paper, copier paper and the like. X-ray film sheets and other sheet materials under certain conditions may exhibit a longitudinal or transverse curl which can interfere with movement of the sheets through the equipment. The curl can be particularly acute if the ambient atmosphere of the equipment has a relative humidity which is substantially lower than that to which the sheets were equilibrated prior to hermetic packaging. In such a case, the layers of photosensitive materials on the upper side of the sheet tend to equilibrate with the conditions in the equipment at a different rate than the layers on the underside of the sheet. This differential response leads to formation of the undesirable curl, which may cause the edges of the sheets to lift as much as a few sixteenths of an inch (0.159 cm) or more.
Various techniques have been tried for improving the dimensional stability of photographic film sheets during changes in ambient relative humidity. Latex polymers have been added to the layers of photosensitive materials. Master rolls of film base and completed film have been subjected to long and complex heat treatment procedures to reduce their tendencies to curl. Hydrophobic layers have been added to the films to reduce their tendency to absorb or release moisture. While these techniques have provided certain improvements in performance, the problem of curl has persisted for sheets of film. A need has existed for a simple, reliable way to dry film sheets to an appropriate degree prior to packaging.